Emergency lighting systems are used in many types of facilities to provide DC battery-powered lighting during periods when the main AC power supply has become temporarily inoperative for some reason. Examples of such facilities include schools, hospitals, government offices, hotels and motels, industrial buildings, multi-unit dwellings, shopping malls, and airports. In many cases, these structures are very large and require that emergency lampheads be placed at several different locations to provide adequate coverage. Fire safety codes require that emergency lighting systems be tested periodically to ensure that they will operate properly during an emergency. With a system employing many separate lampheads at scattered locations, these tests can be laborious and time-consuming to perform. For this reason, various types of self-diagnostic systems have been developed to facilitate the testing procedure.
A typical emergency lighting system consists of a battery for supplying power to one or more lampheads during an AC power loss, a charger for charging the battery from the AC power supply during standby operation, and a relay or other type of switching device for connecting the lampheads to the battery when an AC power loss is detected. When a self-diagnostic system is provided, it generally operates by briefly simulating an AC power outage and checking to be sure that the emergency lampheads illuminate properly. The test may be initiated manually, by depressing a pushbutton or operating a remote control device, or automatically in response to an internal timer. In some cases, an internal control system (such as a microprocessor) automatically carries out a number of different tests in sequence, such as tests for lamp current flow, power transfer from charger to battery, and battery voltage. If one or more of these tests fails, a light-emitting diode (LED) or other type of visual indicator may be illuminated to indicate that maintenance is required. In more sophisticated systems employing central computer monitoring, an indication of test failure may also be produced on a computer display terminal at a central monitoring location.
In some emergency lamphead systems, the battery and charging circuitry are housed in a separate unit which is remote from some or all of the lampheads to which it is connected. When self-diagnostic circuitry is provided, it will ordinarily be located in the central unit rather than in the remote lampheads. This facilitates testing for proper battery and charger operation, but makes it difficult to check for proper operation of the individual lampheads. Problems which can render an individual lamphead inoperable include a defective, burned out or improperly connected lamp, or a wiring problem at the lamphead. Most of these problems can be detected by checking for proper electrical continuity through each lamphead, but this is difficult to accomplish from a central location. The remote lampheads are typically connected to each other and to the central battery and charging unit in a parallel "daisy chain" arrangement, and hence a self-diagnostic circuit located at the central unit cannot perform separate tests on each lamphead to identify a specific lamphead that requires service. Typically, therefore, a central monitoring or diagnostic circuit shows that one of the lampheads is not operating for some reason, but does not specify the identity or location of the inoperative lamphead. It then becomes necessary to place the entire system into emergency mode operation in order to visually identify the lamphead which is not operating.
The problem of checking for proper electrical continuity at remote lampheads is more difficult to solve than might be expected. There is a need to minimize the number of lines or connections between the remote lampheads and the central unit; therefore, the solution does not lie in running a large number of additional wires between the remote lampheads and the central unit to support diagnostic functions. Conversely, the expense and complexity of the self-diagnostic circuitry is ordinarily such that it is not practical to provide the circuitry at each remote lamphead location. Even if this were attempted, the "daisy chain"connections between remote lampheads would give rise to the additional problem of maintaining proper isolation between the self-diagnostic circuits of the individual lampheads, so that the output of each diagnostic circuit will reflect the condition of its associated lamphead without being affected by the condition of other lampheads.